This invention relates to the measuring of the difference in group delay suffered by a signal envelope of a swept measuring carrier frequency with regard to the group delay suffered by a signal envelope of a preferably fixed reference carrier frequency after both signal envelopes have been passed through the same transmission path. Such measurements of frequency dependent group delay distortions are performed over the interesting range of transmitting frequencies of communication paths in order to obtain a measure for the transmission quality and any disturbances, such as cross talk, because of unwanted intermodulations in frequency modulated multiplex communication channels.
Group delay t.sub.g is defined as t.sub.g = db/d.OMEGA., where b is the phase shift of a signal component of the frequency .OMEGA. passed through a transmission path. Virtually all known group delay measuring arrangements are based on the method disclosed by H. Nyquist and S. Brand in "Measurements of Phase Distortion," Bell System Techn. Journal 9 (1930), page 522. According to this method the amplitude of a signal with a carrier frequency .OMEGA. is modulated by a "split" frequency .omega., where .omega. is substantially smaller than .OMEGA. so that a carrier signal with an envelope of the frequency .omega. is generated. This composed signal is passed through a transmission path and is then demodulated to recover said envelope. The phase shift b of this envelope signal is compared to a reference phase derived from the input signal. Then the measured phase shift .DELTA.b has to be scaled to .DELTA..OMEGA. which, as the theory shows, is .omega..
According to the German patents 1 025 072 and 1 050 441 the phase measurement according to the Nyquist method can be further modified in that the amplitude modulated measuring carrier signal and a reference carrier signal are applied to the item under test in a periodically alternating sequence at a given change-over frequency and the reference carrier is, during a part of its period, modulated by an identification signal. The measurement and reference signals are demodulated and alternately applied to a phase sensitive detector. The other input of the phase detector is fed from a voltage controlled oscillator connected in a phase lock loop to the detector output via filter means. Thus, the phase of the oscillator is adjusted to the average phase value of the alternating detector input signals.
According to German Pat. No. 1 294 547, which corresponds to U.S. Pat. No. 3,4l4,809, all auxiliary frequencies required at the transmitting end, namely the split frequency, the indentification frequency and the change-over frequency are derived from a single oscillator and all auxiliary frequencies required at the receiving end, namely the indentification frequency, the change-over frequency and the reference signal are derived from the single frequency controlled oscillator in fixed phase relationship to the received demodulated split frequency as this oscillator is connected in a phase lock loop to the phase detector output. Thereby, the accuracy of the phase measurement is improved.